The corrugated cardboard is composed of a plurality of sheets of cardboard, alternately smooth and corrugated, glued to one another. To produce this type of cardboard, corrugating or “single facer” machines are used, wherein a pair of corrugating rollers, provided with longitudinal grooves that mesh together, form a nip through which a first sheet of cardboard passes, which is corrugated and provided with a glue on the crests of the flutes. The corrugated sheet is then glued to a smooth sheet or “cover”. The product thus obtained can be provided with a second smooth sheet, or cover. Differently, several products coming from a corrugating machine can be superimposed and glued to obtain a corrugated cardboard with several layers.
Corrugating machines of this type are described in EP-A-870598, EP-A-601528, U.S. Pat. No. 6,068,701, EP-A-786329, EP-A-1086805, US-A-20010047850, U.S. Pat. No. 5,415,720, EP-A-734849.
Corrugating rollers are relatively complex components. In fact, they have an internal heating circuit through a heat-carrying fluid, typically steam. The heat-carrying fluid circuit has an internal duct and a series of external ducts, in proximity to the cylindrical surface of the roller, to obtain efficient heat exchange.
Due to their complex nature, currently known corrugating rollers are produced with several components. For example, U.S. Pat. No. 4,917,664 describes a corrugating roller constituted by two head ends on which the shanks or necks to support the roller are provided. The head ends are connected to a hollow cylindrical body, on the outer surface of which ribs or grooves are produced to perform corrugation of the cardboard, and in the cylindrical wall of which ducts are produced parallel to the axis of the roller for circulation of the heat-carrying fluid. The ducts are disposed to allow the heat-carrying fluid to flow alternately in one direction and in the opposite direction.
ES-B-2070726 describes a corrugating roller comprising a interchangeable external cylindrical jacket, on which grooves or corrugations are produced, fitted on a grooved and radially perforated central core. Once the jacket has been mounted on the central core, these grooves form longitudinal ducts for circulation of the heat-carrying fluid. The core is axially perforated to define an inlet duct and an outlet duct of the heat-carrying fluid. It forms, with its ends, the end necks or shanks of the roller.
ES-A-2110871 describes a corrugating roller comprising a hollow cylindrical body, at the ends of which two portions forming the shanks or necks of the roller are inserted. One of the two portions has an inlet duct and an outlet duct for the heat-carrying fluid, which circulates in an interspace with an annular section formed by the inner wall of the hollow cylindrical body and by a pipe coaxial to said body.
EP-B-657275 describes a corrugating roller comprising a hollow cylindrical body, in the wall of which circulation ducts for the heat-carrying fluid are produced. The ducts are fed through head ends forming the necks or shanks supporting the roller. The head ends are inserted into the axial cavity of the cylindrical body.
U.S. Pat. No.5,899,264 and EP-A-1962590 describe a corrugating roller constituted by a hollow cylindrical body, in the wall of which ducts for circulation of the heat-carrying fluid are produced. The fluid is fed through an axial hole produced in one head end of the roller, fixed to the central cylindrical body and passes through a duct coaxial to the cylinder, extending through the hollow cylindrical body thereof, to the opposite head end from which the fluid is distributed radially to the peripheral ducts produced in the cylindrical wall of the body of the roller. Radial holes in the first head end collect the spent heat-carrying fluid and convey it outside.
All the constructional solutions described in the aforesaid documents are complex and costly and in some cases somewhat inefficient from a thermal point of view, as they do not allow efficacious heating of the outer surface of the corrugating roller.